It is still unclear whether, in the male rat, castration and androgen replacement affect the binding characteristics of brain opiate receptors. To clarify this issue, the effects exerted by orchidectomy and testosterone (T) replacement on the subpopulation of brain mu opiate receptors were studied in male rats; testosterone was administered via subcutaneous Silastic capsules. Utilizing 3H-dihydromorphine (a mu receptor ligand) it has been shown that the affinity constant (Ka) of brain mu opiate binding sites, measured in plasma membrane preparations, is not affected by castration. When mu receptor concentrations were measured in individual brains, it was found that gonadectomy and T replacement failed to produce any change in the number of mu opiate receptors. These data suggest that, in male rats, gonadal steroids do not develop their central feedback effects by affecting brain mu opiate receptors.